Liquid droplet ejecting apparatus and liquid droplet ejecting method

ABSTRACT

A liquid droplet ejecting apparatus having: a liquid droplet ejecting head; and a drive pulse generating unit, wherein the head includes: a nozzle; a pressure chamber which communicates with the nozzle; and a pressure applying section which changes a pressure in the pressure chamber, wherein the generated drive pulse is applied to the pressure applying section so as to change the pressure in the pressure chamber to cause the liquid in the pressure chamber to be ejected from the nozzle, and wherein the drive pulse comprises a rectangular expansion pulse which causes expansion and then contraction of the volume of the pressure chamber and in which the pulse width PW of the expanding pulse is set so as to satisfy the following conditional equation, 
     
       
         
           
             
               
                 
                   PW 
                   = 
                   
                     
                       π 
                       - 
                       
                         ( 
                         
                           
                             tan 
                             
                               - 
                               1 
                             
                           
                            
                           
                             1 
                             
                               2 
                                
                               π 
                                
                               
                                   
                               
                                
                               f 
                                
                               
                                   
                               
                                
                               τ 
                             
                           
                         
                         ) 
                       
                     
                     
                       2 
                        
                       π 
                        
                       
                           
                       
                        
                       f 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
         
         
           
             where f represents an acoustic resonance frequency of a pressure wave in the pressure chamber and τ represents a damping time constant of the pressure wave.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid droplet ejecting apparatus anda liquid droplet ejecting method.

2. Description of Related Art

In the liquid droplet ejecting head in which liquid droplets are ejectedfrom a nozzle such as an inkjet recording head (also called recordinghead hereinafter) for recording images using small ink droplets, liquiddroplets are ejected from the nozzle and land on a recording medium suchas recording paper and the like by applying pressure to a pressurechamber.

There are various pressure application methods for applying pressure inthe pressure chamber, and as disclosed in Patent Document 1, one exampleis the type in which ink droplet ejection pressure is obtained by usinga piezoelectric element.

In the past, in the case where the ink droplets were ejected from thenozzle by increasing the pressure in the pressure chamber by expandingand then contracting the volume of the pressure chamber, the pulse widthof the expansion pulse for expanding and then contracting the volume inthe pressure chamber was considered to be capable of ejecting mosteffectively when equal to 1 AL (Acoustic Length), and so this has beenused. (See Unexamined Japanese Patent Application No. 2002-19103publication). The “AL” is a unit of time and 1 AL corresponds to ½ ofthe acoustic resonance period of the pressure chamber.

However, according to the findings of the inventors, the negativepressure wave that is generated by the expansion dampens with thepassage of time when it propagates through the pressure chamber. As aresult, it was determined that when damping of the pressure wave isconsidered, if the pulse width of the expanding pulse is set shorterthan 1 AL to which it is set in the aforementioned prior art, ejectioncan be more efficient.

As is the case in the prior art, when the pulse width of the expandingpulse is set to 1 AL, at the point where the positive pressure exceedsthe maximum (peak) and is decreasing, removing application of theexpansion pulse is carried out and ejection efficiency is reduced.

SUMMARY

The present invention was conceived in view of the aforementionedproblems and the object thereof is to provide a liquid droplet ejectingapparatus and liquid droplet ejecting method which can eject liquiddroplets with higher efficiency.

According to one aspect of the present invention, there is provided aliquid droplet ejecting apparatus comprising: a liquid droplet ejectinghead; and a drive pulse generating unit adapted to generate a drivepulse, wherein the liquid ejecting head includes: a nozzle which ejectsliquid droplets; a pressure chamber which communicates with the nozzle;and a pressure applying section which changes a pressure in the pressurechamber by expanding or reducing a volume of the pressure chamber,wherein the drive pulse generated by the drive pulse generating unit isapplied to the pressure applying section so as to change the pressure inthe pressure chamber and the change of pressure in the pressure chambercauses the liquid in the pressure chamber to be ejected from the nozzle,and wherein the drive pulse comprises a rectangular expansion pulsewhich causes expansion and then contraction of the volume of thepressure chamber and in which a pulse width PW of the expanding pulse isset so as to satisfy the following conditional equation,

$\begin{matrix}{{PW} = \frac{\pi - \left( {\tan^{- 1}\frac{1}{2\pi \; f\; \tau}} \right)}{2\pi \; f}} & (1)\end{matrix}$

where f represents an acoustic resonance frequency of a pressure wave inthe pressure chamber and τ represents a damping time constant of thepressure wave.

According to another aspect of the present invention, there is providedthe liquid droplet ejecting apparatus described above, wherein thedamping time constant τ is not less than 8×10⁻⁶ (sec) and not more than100×10⁻⁶ (sec).

According to still another aspect of the present invention, there isprovided the liquid droplet ejecting apparatus described above, whereinthe drive pulse further comprises a rectangular contraction pulse thatfollows the rectangular expansion pulse and causes contraction and thenexpansion of the volume of the pressure chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the schematic structure of the inkjet recording apparatus.

FIGS. 2( a) and 2(b show the schematic structure of the shear mode typerecording head which is one aspect of the liquid droplet ejecting headand specifically, FIG. 2( a) is a perspective view of a partial crosssection while FIG. 2( b) is a cross-sectional view of the state wherethe ink supply section is loaded.

FIGS. 3( a)-3(c) show the operation of the recording head.

FIG. 4( a) shows the waveform of the drive pulse and FIG. 4( b) is thewaveform showing the pressure changes of the pressure chamber when theexpansion pulse is applied.

FIG. 5( a)-5(c) are explanatory drawings for the time-shared driving ofthe recording head.

FIG. 6 is the timing chart of the driving pulse that is applied to theelectrode of the pressure chamber in each of the phases A, B, and C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a description of the embodiments of the presentinvention, but aspects of this invention are not to be limited by theseembodiments.

The embodiments of the present invention will be described using thedrawings.

FIG. 1 shows the schematic structure of the inkjet recording apparatusused in the liquid droplet ejecting apparatus of this invention. In theinkjet recording apparatus 1, the recording medium P is nipped in theconveyance roller pair 32 of the conveyance mechanism, and then conveyedin the Y direction of the drawing by the conveyance roller 31 that isdriven by rotation using the conveyance motor 33.

The recording head 2 is provided so as to oppose the recording surfacePS of the recording medium P. This recording head 2 is loaded onto thecarriage 5 so that the nozzle surface side opposes the recording surfacePS of the recording medium. The carriage 5 is provided along the guiderail 4 that extends along the width direction of the recording medium Pso as to be moveable back and forth in the X-X′ direction in the drawing(main scanning direction) which is substantially perpendicular to theconveyance direction (sub-scanning direction) of the recording medium Pby a driving unit that is not shown. The recording head 2 iselectrically connected via a flexible cable 6 to the drive pulsegenerating unit 100 (See FIG. 3) which has a circuit for generating thedrive pulse.

The inkjet recording apparatus 1 comprises a control section and amemory section (not shown). The control section is the site whichcontrols the entire inkjet recording apparatus 1 and may for example bea microcomputer comprising a CPU (central processing unit); a memory forstoring programs; and a memory for temporarily storing informationrequired for processing. The control section performs prescribedprocessing by executing the programs stored in memory.

The drive pulse generating unit 100 performs driving by applying a drivepulse to the pressure applying section such a the piezoelectric elementsand the like which are in the pressure chambers of the recording head 2,in order to eject liquid droplets from the nozzle based on instructionsfrom the control section.

The drive pulse comprises the rectangular expansion pulse which causescontraction after the volume of the pressure chamber is expanded, and arectangular contraction pulse which causes expansion after the volume ofthe pressure chamber is contracted following application of theexpansion pulse (see FIG. 4( a)). The pulse width PW of the expansionpulse is set to satisfy the following equation (1) where the acousticresonance frequency in the pressure chamber is f and the time constantfor damping of the pressure wave is τ.

$\begin{matrix}\text{[Equation~~3]} & \; \\{{PW} = \frac{\pi - \left( {\tan^{- 1}\frac{1}{2\pi \; f\; \tau}} \right)}{2\pi \; f}} & (1)\end{matrix}$

The memory section is a memory medium which stores data such as thepulse width PW of the expansion pulse and may take any form such as areadable and writable memory comprising semiconductor memory and thelike or a memory device such as a magnetic disk device or the like.

The memory head 2 moves in the X-X′ direction of the drawing on therecording surface PS of the recording media P with the movement of thecarriage 5 and prescribed inkjet images are recorded by this movementprocess due to ink droplets being ejected.

It is to be noted that 7 is the ink receiver and the recording head 2 isprovided at a waiting position such as the home position when norecording is being done. When the recording head is at this waitingposition and is not in operation for a long period of time, the surfaceof the nozzle of the recording head 2 can be protected by being coveredwith a cap. 8 is also an ink receiver that nips the recording media Pand is provided at a position opposing the ink receiving device 7 andwhen recording is done back and forth in both directions, when theswitch is made between the forward movement and the backward movement,the flown ink droplets are received in the same manner as above.

The liquid droplet ejecting apparatus and liquid ejecting method of thisinvention may use any type of liquid droplet ejecting head provided thatthe liquid droplet ejecting head comprises: a nozzle for ejecting theliquid droplets; a pressure chamber that communicates with the nozzle;and a pressure applying section which changes the pressure of thepressure chamber by expanding or reducing the volume of the pressurechamber. Also, any liquid may be used to fill the pressure chamber. Ashear mode type recording head 2 which is a liquid droplet ejection headusing ink as the liquid for filling the pressure chamber is used in thefollowing description.

In the shear mode type recording head, the partition walls of thepressure chamber are formed of a piezoelectric element which is thepressure applying section and ink is ejected from the nozzle bysubjecting the piezoelectric element to shear deformation.

FIG. 2( a) and FIG. 2( b) show the schematic structure of the shear modetype recording head which is one aspect of the liquid droplet ejectinghead and FIG. 2( a) is a perspective view of a partial cross sectionwhile FIG. 2( b) is a cross-sectional view of the state where the inksupply section is loaded.

It is to be noted that all of the pressure chambers have the samestructure so the alphabet characters for indicating the structure arenot included for the individual pressure chambers and sometimes indicateall of them.

FIGS. 3( a)-3(b) show the operation of the recording heads.

In FIGS. 2( a) and 2(b and FIGS. 3( a) and 3(b), 2 is a recording head,21 is an ink tube, 22 is a nozzle forming member, 23 is a nozzle, 24 isa cover plate, 25 is an ink supply port, 26 is a base plate, 27 is apartition wall, L is the length of the pressure chamber, D is the depthof the pressure chamber, and W is the width of the pressure chamber. Inaddition, the pressure chamber 28 comprises the partition walls 27, thecover plate 24 and the base plate 26.

As shown in FIG. 3( a) and FIG. 3( b), the recording head 2 is the sheartype recording head in which there are a plurality of the pressurechambers 28 that are partitioned between the cover plate 24 and baseplate 26, by a plurality of partition walls 27A, 27B, 27C, and 27D whichare formed from a piezoelectric material such as PZT. In FIG. 3( a) andFIG. 3( b), 3 pressure chambers (28A, 28B, and 28C) which are some ofthe multiple pressure chambers 28 are shown. The end of the pressurechamber 28 (sometimes called nozzle end hereinafter) is connected to thenozzle 23 that is formed on the nozzle forming member 22, and the otherend (sometimes called manifold end) is connected via the ink supply port25 to the ink tank (not shown) by the ink tube 21. In addition, theelectrodes 29A, 29B, and 29C which hang from the top of both partitionwalls 27 to the bottom surface of the base plate 26 are densely formedon the upper surface of the partition walls 27 inside each pressurechamber 28 and each of the electrodes 29A, 29B, and 29C are connected tothe drive pulse generating unit 100.

Next, the method for manufacturing the recording head 2 and componentmaterials will be described.

Two sheets of piezoelectric material 27 a and 27 b are vertically bondedonto the base plate 26 such that the polarization directions areopposite of each other and a diamond blade or the like is used to cutfrom piezoelectric material 27 a which is the upper side, parallelmultiple grooves with the same configuration to form the pressurechambers 28. As a result, the adjacent pressure chambers 28 arepartitioned by the side walls 27 that are polarized in the direction ofthe arrow. Also, the pressure chamber 28 comprises a deep groove portion28 of the outlet port side (left side in FIG. 2) of the pressure chamber28 and a shallow groove portion which gradually becomes shallow as theinlet port side (right side in FIGS. 2 a and 2 b) is approached from thedeep groove portion 28 a.

Each partition wall 27 herein is formed from two sheets of piezoelectricmaterials 27 a and 27 b which have opposite directions of polarity asshown by the arrows in FIG. 3, but the piezoelectric member should be atleast one portion of the partition wall and may be only the 27 a portionfor example.

There are no particular limitations on the piezoelectric material usedfor the piezoelectric material 27 a and 27 b provided that deformationis generated when voltage is applied, and known piezoelectric materialsmay be used. A base plate that is formed from organic material may beused, but a piezoelectric non-metal material is preferable. Examples ofthe base plate formed from a piezoelectric non-metal material include aceramic base plate that is molded by processes such as molding, bakingand the like, or a base plate molded by processes such as coating andlamination. Examples of organic materials include organic polymers andhybrids of organic polymers and inorganic substances.

Examples of the ceramic base plate include, PZT (PbZr0₃-PBTiO₃) thirdcomponent additive PZT and examples of the third component includePb(Mg_(1/3)Nb_(2/3))O₃, Pb(Mn_(1/3)Sb_(2/3))O₃, Pb(Co_(1/3)Nb_(2/3))O₃and the like. In addition, BaTiO₃, ZnO, LiNbO₃, LiTaO₃ and the like maybe used to form the base plate.

Examples of the base plate formed by processes such as coating andlamination include those formed by the sol-gel method, laminated baseplate coating and the like.

A cover plate 24 that is bonded to the upper surface of thepiezoelectric material 27 a using adhesive, so as to extend along allthe pressure chambers 28 and cover the deep groove portion 28 a, and anink inlet port 77 to the inside of the pressure chamber 28 are formed onthe shallow groove 28 b of the pressure chambers 28.

After bonding of the cover plate 24, one nozzle forming member 22 inwhich the nozzle 23 is provided, is bonded using adhesive. As shown inFIG. 2 b, the nozzle 23 of the present embodiment has a taperedconfiguration in which the diameter at the ink outlet port side issmaller that the diameter at the ink inlet port side of the nozzle.

The nozzle diameter refers to the diameter of the front end openingportion at the ink outlet side of the nozzle, and in the case where thecross-section of the opening portion is circular it is the diameter ofthe cross section. It is to be noted that the shape of the cross sectionof the nozzle does not have to be circular and the cross-section mayhave other shapes such as polygonal or star-shaped. It is to be notedthat in the case where the cross-section is not circular, the nozzlediameter is the diameter of a circle with the same surface area as thecross sectional area.

No particular limitations are imposed on the material that can be usedfor the cover plate 24 and the base plate 26, and a base plate may beformed from an organic material but it is preferably formed from anon-piezoelectric non-metal material and the non-piezoelectric non-metalmaterial is preferably at least one selected from alumina, aluminumnitride, zirconia, silicon, silicon nitride, silicon carbide, quartz,and non-polarized PZT. Examples of the organic material include organicpolymers and hybrids of organic polymers and inorganic substances.

In addition, examples of the material used for forming the nozzleforming member include synthetic resins such as polyimide resin,polyethylene naphthalate resin, crystal polymers, aromatic polyamideresin, polyethylene naphthalate resin, polysulfone resin, as well asmetal materials such as stainless steel and the like.

A metal electrode 29 is formed inside each pressure chamber 28 to extendfrom both side surfaces to the bottom surface thereof, and the metalelectrode 29 extends to the rear side surface of the piezoelectricmember 27 a through the shallow portion 28 b. A flexible cable 6 isbonded to each of the metal electrodes 29 via the anisotropicallyconductive film 78 on the rear side surface and the side wall 27 issubjected to shear distortion by applying drive pulses from the drivepulse generating unit 100 to the metal electrodes 29 and the pressure atthe time of deformation causes the ink inside the pressure chamber to beejected from the nozzle 23 that is formed on the nozzle plate 22.

Examples of the metal used to form the metal electrode 29 includeplatinum, gold, silver, copper, aluminum, palladium, nickel, tantalum,titanium, and gold, aluminum, copper, and nickel are preferable in viewof conductive and processing properties and the electrodes are formed byplating, vapor deposition, or sputtering.

As described above, in the shearing mode type recording head 2, pressurechambers 28 are formed on the piezoelectric materials 27 a and 27 b andby merely forming the metal electrodes 29 on the side walls thereof, themain portion of the head can be formed and thus, manufacturing is simpleand because multiple pressure chambers 29 are arranged with a highdensity, this is a favorable form as high resolution image recording canbe performed.

Next the ejection operation will be described.

When a drive pulse is applied from the drive pulse generating unit 100to the electrodes 29A, 29B, and 29C that are densely formed on thesurface of the partition walls 27, ink droplets are ejected from thenozzle 23 due to the operation used as an example in the following. Itis to be noted that the nozzle was not included in FIG. 3.

It is also to be noted that as described above, in the recording head 2,positive and negative pressure is exerted on the ink inside the pressurechamber 28 due to deformation of the partition wall 27 and the partitionwall 27 comprises the pressure applying section.

FIG. 4( a) shows the drive pulse in the liquid droplet ejection methodof an embodiment of this invention and FIG. 4( b) is the waveformshowing the pressure changes of the pressure chamber when the expansionpulse of FIG. 4( a) is applied. In FIG. 4( b), the X-axis is time andY-axis is pressure.

(1) As shown in the state of FIG. 3( a), in the head 2, when theelectrode 29A and the electrode 29C are grounded and a rectangular waveexpansion pulse (positive voltage) in which the pulse width PW is set tosatisfy (1) is applied to the electrode 29B, an electric field isgenerated that is at right angles to the polarization direction of thepiezoelectric materials 27 a and 27 b which forms the partition walls27B and 27C due to the first rise of the pulse (P1). Shift deformationof the joining surface of 27 a, 27 b, and the partition wall occurs andas shown in FIG. 3( b), the partition walls 27B and 27C both deformtoward the outer side and the volume of the pressure chamber 28Bexpands. As a result, negative pressure −P which is lower than thenormal pressure is generated in the ink inside the pressure chamber 28Band the ink is drawn.

It is to be noted that as described above, AL (Acoustic Length) is ½ ofthe acoustic resonance cycle Tc of the pressure chamber. The AL is 1/(2f) and is obtained by measuring the acoustic resonance frequency f ofthe pressure wave in the pressure chamber. The method for measuring theacoustic resonance frequency f of the pressure wave will be describedhereinafter.

The pulse is the rectangular wave of the fixed high voltage wave and inthe case where 0V is 0% and the high voltage wave is 100%, the pulsewidth is defined as the time between the point of 10% of voltage of 0Vfrom the start of voltage rise or the start of voltage fall and thepoint of 10% of the high voltage wave from the start of voltage rise orthe start of voltage fall. Furthermore, the rectangular wave hereinindicates a waveform such that the rise time is between 10% and 90% ofvoltage, and all the rise times are preferably less than ½ AL and morethan ¼ AL.

(2) The negative pressure is transmitted to the pressure chamber withdamping and after normal pressure returns, it inverts to positivepressure and the maximum (peak) positive pressure at t_(max), which isfrom the first application of P1 to the point before 1 AL time elapses,is reached. Thus at this point, when the potential returns to 0 (P2),the partition walls 27B and 27C return from the expansion position tothe middle position shown in FIG. 3 a and a high pressure is exerted onthe ink inside the pressure chamber 28B.

Next, the contraction pulse (negative voltage) comprising rectangularwave is applied. First, as shown in FIG. 3( c), due to the rise (P3) ofthe contraction pulse, the partition walls 27B and 27C deform indirections opposite to each other and the volume of the pressure chamber28B contracts. As a result of this contraction, an even higher pressureis reinforced on the ink in the pressure chamber 28B, and an ink columnprojects from the opening of the nozzle 23.

(3) When 1 AL time elapses, the pressure wave of the ink inside thepressure chamber 28 inverts to negative pressure.

(4) Furthermore, when 1 AL time elapses, the pressure wave inverts topositive pressure and thus the potential returns to 0 (P4) and when thepartition walls 27B and 27C return from the middle position to thecontraction position, the volume of the pressure chamber 28B expands.The pressure wave due to the negative pressure of this expansion and thepressure wave of the positive pressure have a phase gap of 180° and thusthey are offset and cancelled and the pressure wave dampens quickly.After this, the ink column separates and the separated ink flies off asink droplets.

Due to this series of operations, a portion of the ink inside thepressure chamber 28B flies from the nozzle 23 as ink droplets.

As described above, by setting the pulse width of the expansion pulse PWso as to satisfy equation (1), the negative pressure generated at thetime of the expansion pulse rises (P1), propagates the pressure chamber,and inverts to a positive pressure and then the maximum positivepressure is reached at t_(max) (<1 AL) and at the same time, thepositive pressure generated by contraction of the pressure chamber dueto the rise of the expansion pulse (P2) and the fall of the contractionpulse (P3) is applied and these pressures depend on each other to obtainefficient ejection force. As a result, this has the advantage that theink droplet ejection speed is fast.

In the case where the pulse width of the expansion pulse is set to 1 ALas is the case in the prior art, in the region where the positivepressure passes the maximum (peak) and is decreasing (dotted line inFIG. 4( b)), contraction occurs due to the rise of the expansion pulse(P2) and ejection efficiency is reduced.

In addition, in the present embodiment, the pulse width of thecontraction pulse is 2 AL and thus the pressure wave is cancelled and itbecomes possible for driving to occur in a shorter cycle.

As shown in FIG. 4( a), the drive pulse tp is such that if the expansionpulse time is PW, the subsequent contraction pulse time is 2 AL and theearth potential time until the next drive pulse is 2 AL, and 1 drivepulse or 1 cycle is complete in the total time of PW+(2+2)AL. It is tobe noted that the earth potential time does not have to be 2 AL, and maybe suitably set.

In addition, in the drive pulse of FIG. 4( a), the proportion of thedrive voltage Von (V) of the expansion pulse to the drive voltage Voff(V) of the contraction pulse is preferably |Von|≧|Voff|. When therelationship is such that |Von|≧|Voff| in this manner, it has the effectof speeding up the supply of ink to the pressure chamber and thisrelationship is preferable particularly in the case where high frequencydriving of high viscosity ink is performed. It is to be noted that thereference voltage of voltage Von and voltage Voff does not have to be 0.The voltage Von and voltage Voff is the voltage difference between therespective reference voltages.

In the shear mode type inkjet head, the deformation of the partitionwall 27 occurs due to the voltage difference applied to the electrodesprovided at both sides of the wall. As a result, instead of negativepressure being applied to the electrodes in the pressure chamber whicheject ink, the electrodes of the pressure chamber which eject ink aregrounded and thus even if positive voltage is applied to the electrodesof adjacent pressure chambers, they can operate in the same manner.According to the latter method, driving can be done using only positivevoltage and this is favorable in view of power source cost.

Next, time share driving which is an example of the liquid dropletejection method of an embodiment of the present invention will bedescribed.

In the case of driving of the head 2 comprising a plurality of pressurechambers partitioned by partition walls 27 in which at least a portionthereof is formed of a piezoelectric material, when the partition wallsof one pressure chamber 28 performs the ejection operation, because theadjacent pressure chamber 28 is affected, drive control is normallyperformed by forming one group from among the multiple pressure chambersof the pressure chamber 28 that sandwich one or more of each other andare separate, and then they are divided into two or more groups and theink ejection operation is sequentially performed for each group by timesharing.

That is to say, n pressure chambers are grouped into m units where aunit is a prescribed plurality and 1 pressure chamber of each unit isdriven on a cycle of a time interval tp and n pressure chambers aredriven in m cycles. The base cycle T is then formed using the encoderpass D and the carriage is moved back and forth and images are recordedon the recording medium by repeating the base cycle T.

The ejection operation in which m=3 and n=9 will be described furtherusing FIGS. 5( a)-5(c) and FIG. 6. In the example shown in FIGS. 5(a)-5(c), the head comprises 9 pressure chambers which are A1, B1, C1,A2, B2, C2, A3, B3 and C3, and the case where driving is done by thedrive pulse in FIG. 4( a) will be described herein. The timing chart ofthe drive pulse that is applied to the electrodes of the pressurechamber 28 groups A, B, and C at this time are shown in FIG. 6. In FIG.6, the pressure chambers A1-C3 are shown on the Y axis and the time isshown in the X-axis.

As shown in FIG. 6, when driving is done by first applying the drivepulse Pa of the first cycle t1 simultaneously to the 3 pressure chambersA1, A2, and A3, the side walls of these 3 pressure chambers A1, A2, andA3 change simultaneously and ink droplets are ejected from each nozzle.As described above, the first volume of the pressure chamber that ejectsthe ink droplets expands and then suddenly the volume contracts. FIG. 5shows the state where all of the pressure chambers contract. As shown inFIG. 6, when driving is done by applying the drive pulse Pb of thesecond cycle t2 simultaneously to the 3 pressure chambers B1, B2, and B3as is the case below, and then driving is done again by applying thedrive pulse Pc of the third cycle t3 simultaneously to the 3 pressurechambers C1, C2, and C3, the side walls change successively, and in thethree cycles t1, t2, and t3, one round of driving the pressure chambersis done and all of the 9 pressure chambers are driven and the inkdroplets are ejected. Pa, Pb, and Pc are the same drive pulse and theyuse the drive pulse shown in FIG. 4( a) and t1, t2, and t3 are set to beequal to the cycle tp of FIG. 4( a).

All of the pressure chambers are not always actually driven as describedabove, and sometimes only selected pressure chambers are driven to ejectink droplets to form images.

As described above, the inventors discovered that it was possible tosupply a liquid droplet ejecting apparatus and a liquid droplet ejectingmethod capable of ejecting liquid droplets using more effective drivingby setting pulse width PW of the expanding pulse to satisfy the equation(1) given that the acoustic resonance frequency of the pressure wave inthe pressure chamber is f and the time constant for damping of thepressure wave in the pressure chamber is τ. The details are described inthe following.

As mentioned above, the negative pressure −P generated in the pressurechamber by the rising of the expansion pulse (P1) increases in pressurewith the passage of time and after it returns to normal pressure, itinverts to positive pressure and then rises above normal pressure. Afterreaching the maximum positive pressure, pressure decreases and itreturns to normal pressure and these pressure changes are repeated. Atthis time, the amplitude of the waveform that shows the pressure changesdampens in the form e^(−t/)τ which is the time t function (e is the baseof the natural logarithm) and the coefficient of this function t becomethe time constant for the pressure wave.

Given that the acoustic resonance frequency of the pressure wave in thepressure chamber is f (1/sec); the time constant for damping of thepressure wave in the pressure chamber is τ (sec), time is t (sec), andthe circumference ratio is π, the pressure change P(t) is shown byequation (2).

$\begin{matrix}\text{[Expression~~4]} & \; \\{{P(t)} = {{- P}\; ^{- \frac{t}{\tau}}\cos \; 2\pi \; f\; t}} & (2)\end{matrix}$

It is to be noted that the acoustic resonance frequency of the pressurewave in the pressure chamber f can be measured by using a commerciallyavailable impedance analyzer to measure the impedance of thepiezoelectric element of the recording head that is filled with ink andthen obtaining f from the frequency for which the impedance of thepiezoelectric element is reduced by resonance of the ink in the pressurechamber.

The damping time constant τ can be calculated based on equation (2)after measuring the pressure changes P(t) with respect to changes intime.

It is possible to obtain the damping time constant τ by measuring Qvalue of a resonance at a time when measuring the resonance frequency ofthe piezoelectric element with the impedance analyzer.

And it is also possible to measure the resonance period (resonancefrequency) and the damping time directly by measuring vibrations of ameniscus caused by the pressure wave with a displacement gauge.

Given that the amount of phase shift due to damping of the pressure waveis α (rad), P′ (t) which is derived from the above equation is shown byEquation 3.

$\begin{matrix}\text{[Expression~~5]} & \; \\{{P^{\prime}(t)} = {P\; ^{- \frac{t}{\tau}}\sqrt{\left( {2\pi \; f} \right)^{2} + \frac{1}{\tau^{2}}}{\sin \left( {{2\pi \; f\; t} + \alpha} \right)}}} & (3)\end{matrix}$

Here α is shown by equation (4),

$\begin{matrix}\text{[Expression~~6]} & \; \\{\alpha = {\tan^{- 1}\frac{1}{2\pi \; f\; \tau}}} & (4)\end{matrix}$

When the pressure wave reaches the maximum positive pressure,

[Expression 7]

sin(2πft+α)=0   (5)

equation (5) is satisfied and thus the time t_(max) at this time, or inother words the pulse width PW is shown by equation 6.

$\begin{matrix}\text{[Expression~~8]} & \; \\{t_{\max} = {{PW} = {\frac{\pi - \alpha}{2\pi \; f} = \frac{\pi - \left( {\tan^{- 1}\frac{1}{2\pi \; f\; \tau}} \right)}{2\pi \; f}}}} & (6)\end{matrix}$

In this manner, the pulse width PW is a value that is determined basedon the damping time constant of the pressure wave in the pressurechamber τ and the acoustic resonance frequency of the pressure wave f.In the case where there is absolutely no damping of the pressure wave, αis equal to 0 and thus as is evident from equation (6), PW=1/(2 f)=1 ALand this is not problematic in the prior art in which the expansionpulse width is set to 1 AL. However, the time constant of damping of thepressure wave τ is a unique value that is determined by the flow pathsof the recording head, the dimensions of the nozzle, and the propertiesof the ink and propagation of the pressure wave in the pressure chamberalways causes damping. As is evident from equation (6), the pulse widthPW is short to the extent that damping is large, or in other words, tothe extent that the damping time constant τ is small and the shift from1 AL becomes marked. Consequently, the ink ejecting efficiencydecreases. This means that the effects of the present invention aregreater when the damping time constant τ is smaller, but if it issmaller than 8×10⁻⁶ (sec), the effect of the damping time constant τ istoo large and there is the possibility that this may cause an undesiredincrease in the drive voltage, and in the case where the damping timeconstant is between 8×10⁻⁶ (sec) and 100×10⁻⁶ (sec), the effects of thepresent invention are remarkable. If it is larger than 100×10⁻⁶ (sec),PW will be almost the same value as for 1 AL.

In this manner, in order to increase the efficiency of ink ejectioncompared to that of the prior art in which the pulse width of theexpansion pulse is set to 1 AL, the pulse width PW should be set so asto satisfy equation 1.

The pulse width PW that has been set in this manner is stored in thememory section of the inkjet recording apparatus 1. The control sectionof inkjet recording apparatus 1 reads the pulse width PW from the memorysection and controls the drive pulse generating unit 100 and therecording head 2 so that the expansion pulse is generated with thispulse width and applied to the piezoelectric element of the recordinghead 2 and liquid droplets are ejected onto the recording medium P.

It is to be noted that the liquid droplet ejecting apparatus and liquiddroplet ejecting method of the present invention exhibits a remarkableeffect in the case where the viscosity depending on the ink temperatureat the time of ejection is between 10 cp and 50 cp. This is because thistype of ink has a high viscosity and the time constant of damping τbecomes small.

In addition, if the viscosity is too high, it is not easy for the ink tobe smoothly ejected from the nozzle and thus driving voltage increases,so the ink velocity is preferably no greater than 50 cp.

The viscosity can be measured using an oscillating viscosity meter ModelVM-1A-L (manufactured by Yamaichi Electronics).

In the embodiment described above, after the rectangular wave expansionpulse that is set so that the pulse width PW satisfies equation (1) andthe volume of the pressure chambers are expanded by the drive pulse, therectangular wave contraction pulse which causes contraction is appliedimmediately after. The drive pulse of the present invention is notlimited to the drive pulse described above and may use any drive pulseprovided that it has a rectangular expansion pulse set such that thepulse width PW satisfies equation (1).

In the above embodiment, the pressure applying section (partition wall)is formed from a piezoelectric element. In the liquid droplet ejectingapparatus and liquid droplet ejecting method, this case where thepressure applying section is formed from a piezoelectric element ispreferable because it facilitates control by expanding the volume of thepressure chamber.

In addition, in the above embodiment, a rectangular drive pulse that hasa rise time and drop time that are sufficiently shorter a than AL isapplied. By using a rectangular wave, driving is performed that uses theacoustic resonance of the pressure wave more effectively. The inkdroplets are ejected more efficiently than in the method that uses thetrapezoid wave, and thus driving can be done with low drive voltage andthe drive circuit can be designed using a simple digital circuit. Inaddition, there is the advantage that setting of the pulse width iseasy.

In the above embodiment, a shear mode type piezoelectric element whichdeforms using the shearing mode due to application of an electric fieldis used as the pressure applying section. The shearing modepiezoelectric element is preferable because the rectangular drive pulsecan be more effectively used and also because the drive voltage isreduced and more effective driving is possible.

The present invention is however, not to be limited by this embodiment,and for example a piezoelectric element having another form, such as asingle plate type piezoelectric actuator or a longitudinal vibrationtype laminated piezoelectric element may be used. Also,electro-mechanical conversion elements that use electrostatic ormagnetic force may be used.

In the description above, an inkjet recording apparatus was used as theexample of the liquid droplet ejecting apparatus and a recording headfor performing image recording was used as the liquid droplet ejectinghead, but the present invention is not to be limited to these and theinvention may have a wide range of uses as a liquid droplet ejectionapparatus and liquid droplet ejection method which comprises a nozzlefor ejecting the liquid droplets; a pressure chamber that communicateswith the nozzle; and a pressure applying section which changes thepressure of the pressure chamber; and which ejects liquid in thepressure chamber as liquid droplets from the nozzle.

WORKING EXAMPLE

The effects of the present invention will be illustrated based on aworking example.

First, a recording head was prepared under the following conditions. Asshown in FIGS. 1-3, multiple grooves were formed in a base plate made ofPZT to form the side walls and aluminum vapor deposited electrodes wereformed on the side surfaces of each side wall. The recording head wasformed by bonding a cover plate to the upper surface of each side wallusing an adhesive and bonding it to the front end, a nozzle formingmember (thickness 75 μm) into which a nozzle with a diameter of φ20 μmand a taper angle of 6.3° is formed. The nozzle has a circular truncatedcone shape and the taper angle of the nozzle is defined as ½ of thecircular cone shape. And the length of the nozzle is equal to thethickness of the nozzle forming member.

The density of the pressure chambers was set at 180 dpi (141 μm pitch);the width W of each pressure chamber was 85 μm, the length L 5 mm, andthe depth D 200 μm; the ink was a water based ink (viscosity 15 cpmeasured at 25° C.) and the surface tension was 40 dyne/cm measured at25° C.

The acoustic resonance frequency of the pressure wave in the pressurechamber of the recording head f (kHz) was 74. 6 (kHz)=74.6×10³ (1/sec)and the damping time constant τ was 12×10⁻⁶ (sec). These were measuredby the method described above.

From the above, the acoustic resonance cycle Tc of the pressure wave was13.4×10⁻⁶ (sec) and AL was 6.7×10⁻⁶ (sec).

Also, from equation (4) above α=0.176 (rad) and from equation (6)PW=6.3×10⁻⁶ (sec).

As shown in FIG. 4( a) evaluation of the recording head was carried outby applying a driving pulse in which the proportion (|Von|/|Voff|) ofthe drive voltage Von (V) of the expansion pulse to the drive voltageVoff (V) of the contraction pulse (|Von|/|Voff|) is 1, at a voltagewhere the drive voltage is 8.3V, the pulse width PW of the expansionpulse is 6.3×10⁻⁶ (sec) and the pulse width of the contraction pulse andthe length of the earth potential are each 2 AL=13.4×10⁻⁶ (sec) to theelectrodes. Ink droplets were ejected by the recording head being drivenin 3 cycles (every 2 pressure chambers) by time sharing and then theejection speed of 1 suitably selected nozzle was evaluated using themethod below.

The ink droplet ejects 20 ink droplets continuously and the 20^(th) inkdroplet is evaluated.

Measurement of ejection speed: The ink droplet speed at the point wherethe ink droplet had flown approximately 1 mm from the opening of thenozzle was measured by a strobe light measurement which uses a CCDcamera.

COMPARATIVE EXAMPLE

The evaluation was done in the same manner as the working example exceptthat the pulse width of the expansion pulse was set to 1 AL=6.7×10⁻⁶(sec)

The measured ejection speed of the ink droplets was (m/sec) in theworking example and 4.42 (m/sec) in the comparative example and thisconfirmed the effect of the present invention.

Table 1 shows the above described example of the present invention andcomparative example (Example and Comparative example 1), and additionalexamples and comparative examples (Example and Comparative example 2-6).

TABLE 1 Droplet Droplet Example Damping ejection ejection and ChannelNozzle Ink time Drive speed speed Comparative Length Length viscosity ALconstant α tmax voltage (PW = AL) (PW = tmax) example mm μm mPa sec *1 μsec τ (μ sec) rad μ sec Volt m/sec m/sec 1 5 75 15 74.6 6.7 12.1 0.1756.3 8.3 4.42 4.55 2 5 75 3 81.3 6.2 27.5 0.085 6 6.2 7.16 7.21 3 5 75 1078.1 6.4 15.9 0.127 6.1 8.3 6.52 6.55 4 5 75 20 71.4 7 10. 0.211 6.512.4 6.57 6.7 5 5 50 15 81.3 6.2 8.9 0.213 5.7 11.1 7.83 7.91 6 10 75 1535.7 14 13.6 0.317 12.6 9 6.5 6.82 *1: Resonance Frequency kHz

In the examples of the present invention, the ejection speed of inkdroplets at the same drive voltage is larger than in the comparativeexamples and it is clear that the ejection efficiency of the inkdroplets is improved. Conversely, by adjusting the drive voltage suchthat the ejection speed is the same, it becomes possible to lower thedrive voltage.

1. A liquid droplet ejecting apparatus comprising: a liquid dropletejecting head; and a drive pulse generating unit adapted to generate adrive pulse, wherein the liquid ejecting head includes: a nozzle whichejects liquid droplets; a pressure chamber which communicates with thenozzle; and a pressure applying section which changes a pressure in thepressure chamber by expanding or reducing a volume of the pressurechamber, wherein the drive pulse generated by the drive pulse generatingunit is applied to the pressure applying section so as to change thepressure in the pressure chamber and the change of pressure in thepressure chamber causes the liquid in the pressure chamber to be ejectedfrom the nozzle, and wherein the drive pulse comprises a rectangularexpansion pulse which causes expansion and then contraction of thevolume of the pressure chamber and in which a pulse width PW of theexpanding pulse is set so as to satisfy the following conditionalequation, $\begin{matrix}{{PW} = \frac{\pi - \left( {\tan^{- 1}\frac{1}{2\pi \; f\; \tau}} \right)}{2\pi \; f}} & (1)\end{matrix}$ where f represents an acoustic resonance frequency of apressure wave in the pressure chamber and τ represents a damping timeconstant of the pressure wave.
 2. The liquid droplet ejecting apparatusdescribed in claim 1, wherein the damping time constant τ is not lessthan 8×10⁻⁶ (sec) and not more than 100×10⁻⁶ (sec).
 3. The liquiddroplet ejecting apparatus described in claim 1, wherein the drive pulsefurther comprises a rectangular contraction pulse that follows therectangular expansion pulse and causes contraction and then expansion ofthe volume of the pressure chamber.
 4. The liquid droplet ejectingapparatus described in claim 2, wherein the drive pulse furthercomprises a rectangular contraction pulse that follows the rectangularexpansion pulse and causes contraction and then expansion of the volumeof the pressure chamber.
 5. The liquid droplet ejecting apparatusdescribed in claim 1, wherein the pressure applying section comprises ashear mode type piezoelectric element.
 6. The liquid droplet ejectingapparatus described in claim 2, wherein the pressure applying sectioncomprises a shear mode type piezoelectric element.
 7. The liquid dropletejecting apparatus described in claim 3, wherein the pressure applyingsection comprises a shear mode type piezoelectric element.
 8. The liquiddroplet ejecting apparatus described in claim 4, wherein the pressureapplying section comprises a shear mode type piezoelectric element.
 9. Amethod of ejecting liquid droplet from a nozzle of a liquid dropletejecting apparatus having a nozzle which ejects liquid droplets, apressure chamber which communicates with the nozzle, and a pressureapplying section which changes a pressure in the pressure chamber byexpanding or reducing a volume of the pressure chamber, the methodcomprising: applying a drive pulse to the pressure applying section tochange the pressure in the pressure chamber, thereby causing the liquidin the pressure chamber to be ejected from the nozzle, wherein the drivepulse comprises a rectangular expansion pulse which causes expansion andthen contraction of the volume of the pressure chamber and in which apulse width PW of the expanding pulse is set so as to satisfy thefollowing conditional equation, $\begin{matrix}{{PW} = \frac{\pi - \left( {\tan^{- 1}\frac{1}{2\pi \; f\; \tau}} \right)}{2\pi \; f}} & (1)\end{matrix}$ where f represents an acoustic resonance frequency of apressure wave in the pressure chamber and τ represents a damping timeconstant of the pressure wave.
 10. The method described in claim 9,wherein the damping time constant τ is not less than 8×10⁻⁶ (sec) andnot more than 100×10⁻⁶ (sec).
 11. The method described in claim 9,wherein the drive pulse further comprises a rectangular contractionpulse that follows the rectangular expansion pulse and causescontraction and then expansion of the volume of the pressure chamber.12. The method described in claim 10, wherein the drive pulse furthercomprises a rectangular contraction pulse that follows the rectangularexpansion pulse and causes contraction and then expansion of the volumeof the pressure chamber.
 13. The method described in claim 9, whereinthe pressure applying section comprises a shear mode type piezoelectricelement.
 14. The method described in claim 10, wherein the pressureapplying section comprises a shear mode type piezoelectric element. 15.The method described in claim 11, wherein the pressure applying sectioncomprises a shear mode type piezoelectric element.
 16. The methoddescribed in claim 12, wherein the pressure applying section comprises ashear mode type piezoelectric element.